A technique invented to reduce corrosion of steel components on ships could also prevent exotic species from stowing away in the ballast water of cargo ships. The technique, which was designed by Mario Tamburri of the Monterey Bay Aquarium Research Institute in collaboration with Japanese scientists, involves pumping nitrogen gas into ballast tanks, thereby virtually eliminating oxygen in the water and suffocating fish, crabs, mussels, clams, and other critters lurking in the tanks. Ballast tanks, which are used to maintain stability in ships, are a major conveyance of exotic species: A large cargo ship can have up to a dozen such tanks, each the size of a high school gymnasium. When the ship reaches its destination, the water — and everything in it — is released into the sea.